Posted
by
CmdrTaco
on Monday January 17, 2011 @01:19PM
from the whirling-and-twirling dept.

rubycodez writes "Berkeley astronomer Sukanya Chakrabarti has detected perturbations in the gases surrounding our Milky Way and concludes there is a satellite 'Galaxy X' 250,000 light years away that is mostly dark matter, but that may contain dwarf stars visible in infrared. She expects many more such dark matter satellites to the Milky Way to be discovered using her technique."

Assuming it exists at all. There is much circumstantial evidence but some argue no direct proof yet (though NASA believe the have proof). Still this excerpt from NASA seems to imply that dark matter does not interact with matter except through gravity.

"The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. "

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation. Also, I think nearby galactic-sized black holes would probably make for some pretty wicked and obvious gravitational lensing.

Alternatively, the detected mass might be a large number of small black holes. I doubt it, but I'm not an astronomer. Luckily, further observation will give us answers.

I don't think that's the case at all. The only reason we have "dark matter" is because of astronomical observations. That is classic science. Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape.

Actually that's a good working definition of a black hole - if its gravity weren't that strong, it wouldn't be one.

It will have objects orbitting around it like planets orbit stars

Yes...

except at distances far greater than a star would normally hold.

...and no, not necessarily. That depends entirely upon the mass of the hole. The gravitational field of a black hole at a given distance is no different than that generated by a star of the same mass at the same distance; the difference is that the hole is so much smaller that you can get much closer to its centre. That vastly reduces the r in GM/(r^2), thus increasing the maximum gravity that can be experienced.

When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky.

The situation is a little more complicated than that thanks to gravitational lensing, but essentially you're correct - a black hole will block light, while dark matter does not.

First off, the GPP has a decent question. The largest supermassive black holes are on the order of 10^9 solar masses, about the same mass as what was calculated for this satellite galaxy. So, I suppose it's at least plausible that it's a single black hole, if unlikely.

But remember and repeat after me: a black hole has no more gravity than any other object of the same mass. As long as you stay away from the event horizon, that is. You need to rethink your first paragraph with that in mind.

So, how would we tell the difference? Well, an X-ray source from the same location would be a good clue that it's a black hole, which says that it's feeding off of something. You should also be able to tell from the gravitational lensing -- dark matter is incredibly diffuse compared to a black hole. It would still bend light, but not quite in the same way, especially considering the distances involved.

But what about a black circle in the sky? Well, the even horizon for such a black hole has the same diameter as the orbit of Pluto, if I remember right. Detectable, maybe, under the right conditions (but not by Hubble -- you'd need something with about 20x better resolution... if I did the math right, which I probably didn't). But we have to capture it overlapping with some other body, such as a background galaxy. By then you'd be better off looking at the lensing effect, anyway. Here [wikimedia.org] is a classic simulation of what I'm talking about.